My Merkel 140AE 450-400 fires (from memory) the right barrel with front trigger & left barrel by the rear trigger.
I had my good friend whose doing the testing for me check the sears early on with my AR2213SC reloads, no difference in grouping or doubling - grouping was more horizontal than circular.
Best load as it turns out in my rifle is 66gn Reloader 15 & foam wad to take up airspace using 400gn Woodleigh softs, Federal 215 primers & Hornady brass.
Velocity just over 2000fps, Chronographed Hornady Factory loads approx 1940fps.